Effect of the seven-valent conjugate pneumococcal vaccine on carriage and drug resistance of Streptococcus pneumoniae in healthy children attending day-care centers in Lisbon

AIMS: Prospective study to evaluate the impact of the7-valentpneumococcalconjugatevaccine(Prevenar) on the nasopharyngeal (NP)carriageofdrug-resistantStreptococcus pneumoniae(DRPn), byhealthychildrenattendingday-carecenters(ages 6 months-6 years). METHODS: Vaccinees (238children) who receivedvaccineand controls (457children) were followed forcarriageof total S.pneumoniaeand DRPn and for the serotypes and genetic backgrounds of DRPn during 6 consecutive sampling periods between May 2001 and February 2003. RESULTS: We detected no significant differences between vaccinees and the control group in the totalcarriagerate of Pn (average, 68%) or in the frequency ofcarriageof DRPn (average, 38%), including the frequency of penicillin-nonsusceptible strains (average, 24%). In contrast, there was a decline in thecarriageof DRPn withvaccineserotypes which was compensated by the appearance and gradual increase in the frequency of DRPn expressing unusual serotypes (6A, 10A, 15A and 15C, 19A, 23A, 33F) which were not present in thevaccineas well as an increase in nontypable strains. The majority of the DRPn with unusual serotypes showed different pulsed field gel electrophoresis patterns indicating replacement of the original resistant flora by other clonal types ofdrug-resistant bacteria. Antibiotic consumption and the frequency of respiratory tract infections were similar among the vaccinees and controls. CONCLUSIONS: Pneumococcalvaccination did not change the frequency ofcarriageofdrug-resistant strains being the initially dominantvaccineserotypes replaced by others expressing nonvaccine serotypes. Reduction in thecarriageof DRPn may require a combination of theconjugatevaccineand a decrease in antibiotic pressure.